1. Field of the Invention
The present invention relates to a digital-analog converter circuit that generates a ramp signal, a solid-state imaging device, and an imaging apparatus.
2. Background Art
Digital-analog (D/A) converter devices that convert digital signals into analog signals are used in many electronic devices.
For example, CCD (Charge Coupled Device)-type that detects light or MOS (Metal Oxide Semiconductor)-type and CMOS (Complementary Metal-Oxide Semiconductor)-type semiconductor devices are used in an imaging apparatus. An imaging apparatus is an apparatus that converts and reads out a light distribution into electric signals using pixels.
Further, among CMOS-type imaging apparatuses, there is an amplification-type imaging apparatus including pixels having active pixel sensors (APS) with drive transistors for amplification in pixel signal generating parts that generate pixel signals according to signal charge generated in charge generating parts.
In the imaging apparatus, address-control is performed on a pixel unit in which plural unit pixels are arranged, the signals from the individual unit pixels are arbitrarily selected, and thereby, pixel signals output by the individual unit pixels are read out.
For example, in an address-type imaging device in which unit pixels are arranged in a matrix, the pixels include active elements having MOS structures (MOS transistors) or the like for amplification functions of themselves. That is, signal charge (photoelectrons) accumulated in photodiodes as photoelectric conversion elements are amplified by the active elements and output as pixel signals.
The analog pixel signals read out from the pixel unit are generally converted by an analog-digital (A/D) converter device into digital signals and output.
There are various A/D conversion systems of pixel signals in view of circuit size, processing speed, resolution, etc. For example, one of the A/D conversion systems is a method of comparing analog pixel signals with a reference signal, counting time necessary for comparison, and converting the analog pixel signals into digital signals based on a count value at completion of the comparison processing. Such an A/D conversion system is called single-slope integration-type or ramp signal comparison-type.
As imaging devices utilizing the A/D conversion system, there are technologies disclosed in JP-A-2000-152082, JP-A-2002-232291, and JP-A-2007-59991, for example.
In the A/D conversion system, the reference signal is a saw-tooth signal (ramp signal) that monotonously changes, and a D/A converter circuit may be used for generating a ramp signal.